The expression of a variety of cell surface antigens were studied on normal bronchial epithelial cells, small cell lung carcinomas, mesothelial cells, mesotheliomas, as well as bronchial epithelial cells, fibroblasts and mesothelial cells transfected with T antigen. The monoclonal antibodies used to study these cell surface antigens defined the determinants normally expressed on a variety of different cell types, mainly those cells of hematopoietic and lymphoid origin. Small cell lung carcinoma cell lines and freshly explanted tumor expressed antigens defined by monoclonal antibodies MY4 and MY9, as well as certain monoclonal antibodies that detect antigens usually associated with the B cells or, in some cases, epithelial cells. In contrast, normal bronchial epithelial cells did not express the MY4 and MY9 antigens. Normal bronchial epithelial cells had been transfected with the H- ras oncogene also expressed in the MY9 antigen. The expression of MHC class I antigens was variable, while MHC class II antigen expression was also variable, but low in these cell lines. Normal mesothelial cells expressed low levels of all of the myeloid- associated antigens. Mesotheliomas varied in their expression of these antigens and there was no consistent pattern which appeared to identify mesotheliomas. In mesothelial cells transfected with T antigen, MY7 and MY4 increased, while MY9 was expressed at variable levels. These findings indicate that bronchial epithelial cells as well as mesothelial cells, both normal and malignant, may express cell surface antigens that are commonly found on other cells types and that expression of certain surface antigens do not necessarily indicate the origin of a malignant cell.